A cell therapeutic agent is a drug used for the purpose of preventing or treating a specific disease through changing characteristics of cells by a method of proliferating or selecting cells ex vivo in order to restore functions of cells and tissues, and has recently received much attention in the fields of refractory diseases and regenerative medicines. Mesenchymal stem cells are multipotent stem cells that have self-renewal capacity and can differentiate into various lineages. Mesenchymal stem cells are also called “mesenchymal progenitor cells”. Mesenchymal stem cells can differentiate into bones, fats, cartilages, nerves, muscles, bone marrow stromal cells, etc. according to conditions, and therefore, they have various therapeutic efficacies. Mesenchymal stem cells are a kind of adult stem cells, and may be mainly isolated together with hematopoietic stem cells from bone marrow. Mesenchymal stem cells are characterized by adhering to culture dishes, unlike hematopoietic stem cells that are floating in culture. Mesenchymal stem cells isolated and cultured under the culture conditions have been used in various experiments and/or clinical applications. However, mesenchymal stem cells have been isolated mainly from bone marrow, fat, or cord blood to be used in study, and during the extraction and isolation of bone marrow-derived mesenchymal stem cells, donors may suffer from pains and there is a disadvantage that a separation efficiency of mesenchymal stem cells is low.
Meanwhile, a placenta is a tissue discarded after birth, and therefore, is easy to obtain, and is an organ where many different kinds of adherent cells exist. Various cells such as mesenchymal stem cells, decidua cells, trophoblast cells, amniotic cells, endothelial cells, etc. are present in portions of the placenta. Zhang et al. disclose a method of isolating mesenchymal progenitor cells from placenta and characteristics of the isolated mesenchymal progenitor cells (Experimental Hematology 32 (2004) 657-664). According to this paper, amniotic sac and decidua are removed from a placenta, and then the placenta is washed with a phosphate buffered saline, and an irrigating solution and a culture solution are allowed to flow through arterial-vein circuit to remove residual blood from the tissues. The tissues are immersed in the culture solution for 12 hours to 24 hours, and mononuclear cells are obtained using a Ficoll density gradient and resuspended in a fetal bovine serum-containing medium, thereby obtaining mesenchymal progenitor cells. This method may be performed at a laboratory scale, because it requires complicated procedures, including Ficoll density-gradient separation. Moreover, since mesenchymal stem cells are cultured in the placenta itself for a long time, mixing of mononuclear cells present in the placenta may be caused. In addition, it is difficult to stably isolate/purify a large amount of healthy mesenchymal stem cells, making it difficult to clinically apply the mesenchymal stem cells. Furthermore, since the placenta free from amniotic sac and decidua is used in the method, the purity of mesenchymal progenitor cells may be lowered since the mesenchymal progenitor cells may be mixed with other cells derived from placental villi. Stable supply and acquisition of a sufficient number of cells which may show efficacy enough to be used as therapeutic agents are a prerequisite for the study of cell therapeutic agents. There is an urgent need to study methods of preparing cell populations applicable not only to existing stem cells but also to treatment and regeneration.
Considering ethical limitations to the research and utilization regarding isolation of cells which may be used as cell therapeutic agents, the limited number of cells to be isolated, and the types of cells that can be isolated from limited single tissues, it is very important to establish a method of isolating cells having excellent therapeutic effects which may be used as a cell therapeutic agent. However, as described above, the known method of isolating placenta-derived cells has a disadvantage of a low separation efficiency, and accordingly, there is a demand for a method of isolating new adherent cells which may be applicable to cell therapeutic agents while satisfying both proliferation capacity and differentiation capacity of placenta-derived cells, as an alternative to the known method.